This invention relates to an amplifier circuit comprising an input terminal, a series arrangement of a first transistor of a first conductivity type and a second transistor of a second conductivity type both arranged as diodes, and a series arrangement of a third transistor of the first conductivity type and a fourth transistor of the second conductivity type, each transistor having a first and a second main electrode and a control electrode, the control electrodes of the first and the second transistor being interconnected, the first main electrode of the first transistor being coupled to the control electrode of the fourth transistor, the first main electrode of the second transistor being coupled to the control electrode of the third transistor, and the intercoupled first main electrodes of the third and the fourth transistor being also coupled to an output terminal.
An amplifier circuit of this type is known from the book "Analysis and design of analog integrated circuits" by P. R. Gray and R. G. Meyer, John Wiley, New York, 1984, page 335 and is intended to be used as a complementary source or emitter follower output stage in a class AB amplifier. Thus, the first main electrode is the emitter if bipolar transistors are concerned and the source if unipolar transistors (such as MOS transistors and JFETs) are concerned. The second main electrode (that is to say, the collector or drain) of the first transistor is coupled to the associated control electrode of this transistor. The second main electrode of the second transistor is coupled to its associated control electrode. The control electrodes of the first and the second transistor are interconnected. The quiescent output current I.sub.out through the third and the fourth transistor, which operate as output transistors, and the quiescent input current I.sub.in through the first and the second transistor are in the ratio of I.sub.out =I.sub.in /n in which for bipolar transistors n is the emitter surface ratio A.sub.E of the first transistor with respect to the third transistor and of the second transistor with respect to the fourth transistor (hence n=A.sub.E1 /A.sub.E3 =A.sub.E2 /A.sub.E4) and in unipolar transistors n, is the W/L ratio and is (n=(W/L).sub.1 : (W/L).sub.3 =(W/L).sub.2 : (W/L).sub.4 in which W is the channel width and L is the channel length.
In some uses of the amplifier circuit it is necessary that the quiescent output current I.sub.out be small as compared with the input current I.sub.in. This means that n must be large. This implies that the first and second transistors, arranged as diodes, must be larger than the third and the fourth transistors. The third and fourth transistors are large anyway because they must be able to supply the large output current required for a class AB amplifier.
If integrated, the amplifier circuit thus realized occupies an undesirably large part of the chip surface area. Moreover, the first and second transistors then have a large parasitic capacitance with respect to the substrate.
To obviate these problems it is known to decrease the voltage drop across the first and the second transistor, for example,
(a) by omitting one of the two transistirs, see, for example, the book by Gray and Meyer, page 410,
(b) or by arranging negative feedback resistors in the output circuit constituted by the third and the fourth transistor, see, for example, U. Tietze and Ch. Schenk, Halbleiterschaltungstechnik, Springer Verlag 1980, page 350.
All of these solutions have their specific drawbacks. A major drawback of the first-mentioned solution is that the symmetry in the amplifier circuit is lost. Spreads in the threshold voltage of the transistor whose transistor diode of the corresponding conductivity type has been omitted then influence the quiescent output current I.sub.out, which is undesirable.
A drawback of the second solution is that the resistance must be large so that a large chip surface area is required in the case of integration and moreover the output impedance of the amplifier circuit increases.